Detection of persistent VHE gamma-ray emission from PKS 1510–089 by the MAGIC telescopes during low states between 2012 and 2017

¹ Inst. de Astrofísica de Canarias, Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
² Università di Udine, 33100 Udine, Italy
³ INFN Trieste, 34149 Trieste, Italy
⁴ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁵ ETH Zurich, 8093 Zurich, Switzerland
⁶ Università di Padova, 35131 Padova, Italy
⁷ INFN Padova, 35131 Padova, Italy
⁸ Technische Universität Dortmund, 44221 Dortmund, Germany
⁹ Croatian MAGIC Consortium: University of Rijeka, 51000 Rijeka; University of Split – FESB, 21000 Split; University of Zagreb – FER, 10000 Zagreb; University of Osijek, 31000 Osijek and Rudjer Boskovic Institute, 10000 Zagreb, Croatia
¹⁰ Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata 700064, India
¹¹ Max-Planck-Institut für Physik, 80805 München, Germany
¹² Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA, Rio de Janeiro, RJ, Brasil
¹³ Unidad de Partículas y Cosmología (UPARCOS), Universidad Complutense, 28040 Madrid, Spain
¹⁴ University of ᴌódź, Department of Astrophysics, 90236 ᴌódź, Poland
¹⁵ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹⁶ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra, Barcelona, Spain
¹⁷ Università di Siena, 53100 Siena, Italy
¹⁸ INFN Pisa, 56127 Pisa, Italy
¹⁹ Università di Pisa, 56126 Pisa, Italy
²⁰ Universität Würzburg, 97074 Würzburg, Germany
²¹ Finnish MAGIC Consortium: Tuorla Observatory (Department of Physics and Astronomy) and Finnish Centre of Astronomy with ESO (FINCA), University of Turku, 20014 Turku, Finland; Astronomy Division, University of Oulu, 90014 Oulu, Finland
²² Departament de Física, and CERES-IEEC, Universitat Autónoma de Barcelona, 08193 Bellaterra, Spain
²³ Universitat de Barcelona, ICCUB, IEEC-UB, 08028 Barcelona, Spain
²⁴ Japanese MAGIC Consortium: ICRR, The University of Tokyo, 277-8582 Chiba, Japan; Department of Physics, Kyoto University, 606-8502 Kyoto, Japan; Tokai University, 259-1292 Kanagawa, Japan; RIKEN, 351-0198 Saitama, Japan
²⁵ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
²⁶ Humboldt University of Berlin, Institut für Physik, 12489 Berlin, Germany
²⁷ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
²⁸ Port d’Informació Científica (PIC), 08193 Bellaterra, Barcelona, Spain
²⁹ INAF-Trieste and Dept. of Physics & Astronomy, University of Bologna, Italy
³⁰ INAF, Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
³¹ Università dell’Insubria, Dipartimento di Scienza ed Alta Tecnologia, Via Valleggio 11, 22100 Como, Italy
³² INAF-Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, LC, Italy
³³ Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA
³⁴ Tuorla Observatory, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
³⁵ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
³⁶ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
³⁷ Aalto University Department of Electronics and Nanoengineering, PO Box 15500, 00076 Aalto, Finland
³⁸ Tartu Observatory, Observatooriumi 1, 61602 Tõravere, Estonia
³⁹ Instituto de Radio Astronomía Millimétrica, Avenida Divina Pastora, 7, Local 20, 18012 Granada, Spain
⁴⁰ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain
⁴¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel, 69, 53121 Bonn, Germany

Received: 12 June 2018
Accepted: 31 August 2018

Abstract

Context. PKS 1510–089 is a flat spectrum radio quasar strongly variable in the optical and GeV range. To date, very high-energy (VHE, > 100 GeV) emission has been observed from this source either during long high states of optical and GeV activity or during short flares.

Aims. We search for low-state VHE gamma-ray emission from PKS 1510–089. We characterize and model the source in a broadband context, which would provide a baseline over which high states and flares could be better understood.

Methods. PKS 1510–089 has been monitored by the MAGIC telescopes since 2012. We use daily binned Fermi-LAT flux measurements of PKS 1510–089 to characterize the GeV emission and select the observation periods of MAGIC during low state of activity. For the selected times we compute the average radio, IR, optical, UV, X-ray, and gamma-ray emission to construct a low-state spectral energy distribution of the source. The broadband emission is modeled within an external Compton scenario with a stationary emission region through which plasma and magnetic fields are flowing. We also perform the emission-model-independent calculations of the maximum absorption in the broad line region (BLR) using two different models.

Results. The MAGIC telescopes collected 75 hr of data during times when the Fermi-LAT flux measured above 1 GeV was below 3  ×  10⁻⁸ cm⁻² s⁻¹, which is the threshold adopted for the definition of a low gamma-ray activity state. The data show a strongly significant (9.5σ) VHE gamma-ray emission at the level of (4.27 ± 0.61_stat)  ×  10⁻¹² cm⁻² s⁻¹ above 150 GeV, a factor of 80 lower than the highest flare observed so far from this object. Despite the lower flux, the spectral shape is consistent with earlier detections in the VHE band. The broadband emission is compatible with the external Compton scenario assuming a large emission region located beyond the BLR. For the first time the gamma-ray data allow us to place a limit on the location of the emission region during a low gamma-ray state of a FSRQ. For the used model of the BLR, the 95% confidence level on the location of the emission region allows us to place it at a distance > 74% of the outer radius of the BLR.